This is a Continuation of German Patent Application, DE 199 53 586.8 with a foreign filing date of Nov. 08, 1999, the disclosure of which is incorporated into this application by reference.
The invention relates to an analytical balance with a balance scale, with a wind guard encircling the balance scale, and with an apparatus for generating an ionized stream of air for the dissipation of electrostatic charges on the weighed goods.
An analytical balance of this type is known from the German Patent Specification 705 715.
Several spikes, which stand vertically upwards and are connected to a high-voltage source, are located in a discharge space which is disposed below the weighing space and is connected to it by a sieve. The sieve acts therein as a large-surface counterelectrode. Thereby a so-called electric wind directed upwards is generated which is intensified by the dischargers"" development of heat.
This vertical stream of air, which strikes the balance scale from below, shifts the zero point of the balance, thereby causing measurement errors. Furthermore, many ions are already discharged on the grounded metallic balance scale so that, above the balance scale, only a small part of the ions generated is still available for draining the electrostatic charge of the weighed goods.
Furthermore, it is known to dissipate electrostatic charges with an external ionization blower. Such external ionization blowers are, for example, described in U.S. Pat. No. 2,264,495 and U.S. Pat. No. 4,689,715. Therein a stream of air is generated by a blower, and this stream of air is ionized at the same time by high-voltage electrodes. Using these external ionization blowers, weighed goods can be blown on first outside the wind guard of the balance with the stream of air from the ionization blower and electrically discharged and then be brought into the balance and onto the balance scale. Alternatively, the weighed goods can be blown on by the ionization blower while on the balance scale, with the wind guard open, and electrically discharged.
Both processes are complicated to manage. In particular, if the weighed goods are insufficiently discharged in the first attempt, the wind guard must be reopened, and the weighed goods must be removed and positioned in front of the ionization blower (in the first method above), or the ionization blower must be aligned to the balance scale (in the alternative method above), and finally the wind guard must subsequently be reclosed. Because these operations require a large number of hand movements, any operator of the balance will take steps to avoid this repetition, such as running the ionization blower for too long during the first time-attempt. As a result, the weighing time is lengthened in most cases. Furthermore, blowing into the weighing space encircled by the wind guard, in the case of the latter, alternative method above, substantially disturbs the thermal equilibrium within the wind guard, so that thermal equilibrium is restored only gradually.
According to one object of the invention, it is sought to provide a balance scale having improved dissipation of the electrostatic charges of the weighed goods.
This and other objectives are achieved by the invention for a balance scale of the aforementioned type by providing an apparatus for the generation of an ionized stream of air that includes a blower. On at least one boundary surface of the weighing space, the blower draws in air from the weighing space. The blower also recycles the air drawn in from the weighing space back into the weighing space via at least one blowing outlet opening at another point.
In this manner, the invention brings a much greater number of ions into the vicinity of the weighed goods, compared to a balance scale according to German Patent Specification 705 715 discussed above, in order to achieve a rapid dissipation of the electrostatic charges. Furthermore, by suitably directing the stream, the vertical component of the stream of air in the vicinity of the balance scale can be minimized, thereby minimizing the effect on the weighing operation. By turning off the blower during the weighing, the effect on the weighing is further reduced. Thus, the invention provides a scale that makes precision weighing possible.
The analytical balance of the invention greatly simplifies handling compared to use of an external ionization blower. Beyond the normal placement of the weighed goods onto the scale, only the ionization apparatus must be started, for example by keystroke. The ionization apparatus according to the invention is operated with the wind guard closed and blows no air from the surrounding environment into the weighing space, which external air could have a different temperature. Instead, the ionization apparatus of the present invention circulates only the air contained within the weighing space.
An especially reliable dissipation of positive as well as negative electrostatic charges is obtained if the apparatus for generating an ionized stream of air generates positive as well as negative ions. For this purpose, the ionization apparatus advantageously has at least two point electrodes which have voltages of opposing polarity.